Ivermectin is a semisynthetic, anthelmintic agent derived from the avermectins, a class of highly active broad-spectrum anti-parasitic agents isolated from the fermentation products of Streptomyces avermitilis. Ivermectin is a mixture containing at least 90% 5-O-demethyl-22,23-dihydroavermectin A1a and less than 10% 5-O-demethyl-25-de(1-methylpropyl)-22,23-dihyro-25-(1-methylethyl) avermectin A1a, generally referred to as 22,23-dihydroavermectin B1a and B1b, or H2B1a and H2B1b, respectively. Ivermectin is described in U.S. Pat. No. 4,199,569. The structural formulas are:

The compound selectively binds with high affinity to glutamate-gated chloride ion channels which occur in invertebrate nerve and muscle cells, leading to an increase in the permeability of the cell membrane to chloride ions with hyperpolarization of the nerve or muscle cell, resulting in paralysis and death of the parasite. The selectivity of the compound is attributable to the facts that some mammals do not have glutamate gated chloride channels and that the compound has a low affinity for mammalian ligand-gated chloride channels. Ivermectin does not readily cross the blood brain barrier in humans.
Ivermectin is active against various life-cycle stages of many but not all nematodes. It is active against the tissue microfilariae of Onchocerca volvulus but not against the adult form. Its activity against Strongyloides stercoralis is limited to the intestinal stages.
Ivermectin is commercially available as STROMECTOL® for eradication of Strongyloides stercoralis, which causes strongyloidiasis, and Onchocerca volvulus, which causes onchocerciasis. Ivermectin is also available as MECTIZAN® for eradication of Onchocerca volvulus and Wuchereria bancrofti. Ivermectin has a plasma half-life of about 15–20 hours. Its metabolite has a plasma half life of about 3 days.
The recommended dosage for treating strongyloidiasis is a single oral dose designed to provide approximately 200 ug of ivermectin per kg of body weight. Additional doses are generally not necessary in order to eradicate infection.
The recommended dosage for treating onchocerciasis is a single oral dose designed to provide approximately 150 ug of ivermectin per kg of body weight. The most commonly used dose intervals in mass distribution campaigns is 12 months. However, retreatment of individuals may be considered at intervals as short as 3 months. Clinical trials have demonstrated efficacy and tolerability that effectively reduces the dermal microfilarial density to near zero after one month and to successfully maintain a low microfilarial level for up to 12 months.
Ivermectin is also used to treat microfilaremia in patients with lymphatic filariasis caused by Wuchereria bancrofti. Ivermectin has not been shown to have any activity against adult worm of any species of Filarioidea causing lymphatic filariasis, in tropical pulmonary eosinophilia syndrome, or in lymphadenitis or lymphangitis associated with lymphatic filariasis. Ivermectin is recommended for use in treating microfilaremia in lymphatic filariasis in patients for whom there may be an increased risk of adverse experiences with the use of other microfilaricides, such as in populations of patients who are, or are likely to be co-infected with Onchocerca volvulus. The recommended dosage for mass distribution for the treatment of microfilaremic in lymphatia filariasis is a single oral dose of approximately 150 to 200 ug/kg once every 6 months. In endemic areas where treatment can only be administered once every 12 months, a dose of approximately 300 to 400 ug/kg is recommended. These doses are based on studies conducted in patients in Africa, Asia, South America and the Caribbean. Ivermectin was compared with diethylcarbamazine in some of these studies. Overall incidences of adverse experiences for an amicrofilaremic population in mass treatment programs were 1%. The following is a list of the more common adverse experiences reported in studies of microfilaremic patients in the literature: fever, headache, myalgia, asthenia/weakness, cough, anorexia, chills, lethargy, arthralgia, nausea, diaphoresis, sore throat, abdominal pain, light-headedness, malaise epigastric pain, postural hypotension, lung function alterations, dizziness, body pain, gastralgia, chest pain, fatigue, respiratory adverse experiences, testicular tenderness, and ascaris expulsion/elimination of worms. The frequency and intensity of adverse experiences are probably related to the pretreatment microfilarial density. Laboratory abnormalities included eosinophilia, liver function abnormalities and hematuria.
Ivermectin has been used to treat head lice (Pediculosis capitis). Glaziou et al. Trop. Med. Parasitol. 45 (1994) 253–254 describe a study in which 26 patients each received a single oral 200 ug/kg dose. The results showed an effectiveness of ivermectin 200 μg/kg single dose against head lice. The authors suggested a second dose on the tenth day to prevent reinfestation from others in the population with head lice. The second dose was not suggested as part of the treatment regimen for the initial infestation.
Youssef et al. Amer. J. Trop. Med. Hyg. 53(6) 1995 pp. 652–653 describe a method of topical application of ivermectin to treat head lice. Ivermectin was found to have an absolute curative effect after a single topical application.
Dunne et al. Trans. R. Soc. Trop. Med. Hyg. 85: 550–551 describe a study in which a single oral dose of 100–200 ug/kg of ivermectin was administered to patients with head lice. They reported a significant, but not absolute, effect on head lice infestation.
U.S. Pat. No. 6,262,031 describes a method for treating head lice infestation by orally administering to the patient, over a period of time of about one week, a total amount of ivermectin of between about 400 ug/kg and 1200 ug/kg.